1. Field of the Invention
The present invention relates to a hot-rolled steel sheet that is suitable for steel pipes, tubes and columns for architecture and civil engineering, electric resistance welded tubes for oil wells, and other general structural materials.
2. Description of the Related Art
Hot-rolled steel sheets that are used as architectural tubes and columns must be strong and tough. Hot-rolled steel sheets that are formed into electric resistance welded tubes must be resistant to "sour fluids", i.e. wet hydrogen sulfide environments.
A conventional method of producing hot-rolled steel sheets having the requisite strength and toughness includes a strengthening step by fining the micro structure achieved by heat treatment with working, e.g. "a thermo-mechanical control process (TMCP)," as disclosed in Japanese Laid-Open Patent No. 62-112,722, Japanese Examined Patents Nos. 62-23,056 and 62-35,452. The conventional method also includes a quenching or controlled cooling step subsequent to the hot-rolling step.
However, conventional methods of producing strong and tough hot-rolled steel sheets are subject to the following problems:
1) The excessive fining of grains, such as TMCP, inevitably increases the yield ratio, i.e., yield strength/tensile strength. Conventional methods, therefore, do not provide the low yield ratio required to prevent buckling and unstable ductile fracture.
2) The sheet cannot be deformed in the thickness direction during the rolling step in the TMCP. Some inhomogeneity in the thickness direction, therefore, occurs in the material. The controlled cooling causes inhomogeneity in the rolling direction of the materials, which makes it hard to control the material quality. Accordingly, the conventional method creates some inhomogeneity in both the thickness and the rolling directions.
3) The conventional TMCP requires a higher rolling reduction at a lower temperature to prevent the formation of austenite crystal grains, and to provide a strong and tough material. This requirement increases the load of the hot-rolling line, and limits the upper size of the hot-rolling material.
4) Strengthening elements used in the conventional TMCP, such as manganese, vanadium, molybdenum, significantly affect material properties, i.e. increased hardenability, increased hardness at the weld section, and decreased toughness at the weld section due to martensite islands generated therein. Therefore, it is difficult to achieve high strength by the TMCP while maintaining satisfactory weld properties.